I’ve had my share of marbled cakes and always felt that the chocolate part was usually a bit on the dry side, but not here: this cake is sheer perfection and to be honest with you the icing is not even necessary.

Preheat the oven to 180°C/350°F. Butter two 20x10x5cm (8x4x2in) loaf pans, line with baking paper and butter the paper as well.
In the large bowl of an electric mixer, cream butter, sugar and vanilla until light and fluffy. Add the whole eggs and egg yolks in 2 or 3 additions, scraping the sides of the bowl after each addition. Beat to incorporate. Sift the flour, baking soda and salt into the batter, add the sour cream and fold with a rubber spatula until batter is evenly blended.
Measure out 2 cups of the batter into another bowl. Add the chocolate and mix to combine. Spoon both vanilla and chocolate batters into prepared pans in layers, alternating spoonfuls of vanilla and chocolate. To create marbling, run a table knife (or wooden skewer) through the batters in a swirling motion.
Bake for about 45 minutes or until a toothpick inserted in the center of the cakes comes out clean. Cool in the pans over a wire rack for 5 minutes, then very carefully unmold, remove the paper and let cool completely on the rack.

Icing: in a small bowl, whisk together the confectioners' sugar and milk. The mixture should have the consistency of thin sour cream. Add the melted chocolate and whisk until glaze is smooth. If it is too thin, add more sugar, 1 tablespoon at a time; if too thick, add more milk, 1 teaspoon at a time. Immediately pour over the cake.

* homemade sour cream: to make 1 cup of sour cream, mix 1 cup (240ml) heavy cream with 2-3 teaspoons lemon juice in a bowl. Whisk until it starts to thicken. Cover with plastic wrap and leave at room temperature for 1 hour or until thicker (I usually leave mine on the counter overnight – except on very warm nights – and it turns out thick and silky in the following morning; refrigerate for a creamier texture)

Hi Patricia, thank you for the recipe. I baked it yesterday in a bundt pan and this cake is moist and tender and a keeper. I reduced the sugar for the cake batter (2 tbsp). The icing was abit thin (runny) - I would reduce it to 1-1.5 tbsp next time. Overall, a really fantastic recipe! Thanks.